The same group is believed to have been behind attacks on the Co-op and Harrods, but it was M&S that suffered the biggest impact.
TCS says it has over 607,000 employees across the world and is the lead sponsor of three prestigious marathons - New York, London and Sydney.
On its website, TCS said it worked with M&S on Sparks, its customer reward scheme.
In 2023, TCS and M&S won the Retail Partnership of the Year award, external at the Retail Systems Awards.
TCS has a portfolio of well-known clients, external including the Co-op, according to its website., external
There is no indication if the internal probe is also looking at the hack on the Co-Op.
TCS also counts easyjet, Nationwide and Jaguar Land Rover among its many clients.
Earlier this week, M&S chief executive Stuart Machin said: "Over the last few weeks, we have been managing a highly sophisticated and targeted cyber-attack, which has led to a limited period of disruption."
In a media call on Wednesday, he did not respond to a question about whether the company had paid a ransom as part of the process.